A cheap and simple way of reduction is to use Meccano gears (plastic gears may also be suitable).
The shaft is divided inside the brass sleeve, under the oil hole, giving the ESSENTIAL independent control.
Mine had a paddle speed of 300 rpm which was a bit of overkill, but made manoeuvring very easy. The geared load on the motors mean that they run very cool so no additional cooling was needed.
This method also saves on the cost of a sound unit - you can't hear it for the gear noise anyway! :)
If I were to do a paddle drive again, I would also use NON-feathering paddles! The paddle going forwards pulls the hull down on that side, whilst the one going in reverse lifts the hull, this causes the boat to list excessively and spills the Captains tea!
Cheers
Danny